1. Field of the Invention
This invention relates to a safety arrangement for fork trucks, comprising a collision preventer which initiates an emergency stopping in response to a contact with or detection of an obstacle.
2. Description of the Prior Art
In safety arrangements used, e.g., in conjunction with U-shaped bumpers on drawbar vehicles it is known to use a bumper which has such a compliance or displaceability that the spring excursion exceeds the stopping distance of the vehicle in the run-up direction even when the vehicle is loaded.
Such concept may be adopted in the present safety arrangement.
The term fork truck covers vehicles having fork blades which can be supported on the ground or floor by supporting rollers and can be moved under pallets and are movable in height only by a short distance to raise the load, as well as vehicles comprising a mast on which a load carrier, in most cases a load-carrying fork, is movable in height by means of a lifting carriage.
In that respect the term fork covers also other load-carrying members.
Various types of collision preventers are known. In that context it is emphasized that the invention is applicable to driver-controlled and to driverless vehicles. Vehicles of the latter type which are provided with carrying forks are usually operated in such a manner that the truck with the body and power unit moves ahead and the load is disposed or handled at the rear end. It is known to provide such trucks with safety arrangements for preventing collisions. In accordance with the concept outlined hereinbefore such safety arrangements are designed to stop the truck on a distance which is so short that the truck and an obstacle will not collide. In the design of an emergency stopping device the stopping distance of a vehicle which carries a load is also taken into account.
Such collision-preventing safety arrangements may comprise, e.g., in accordance with European Pat. No. 0 192 940, a confining structure, which can be deflected or compressed in several directions. In accordance with European Pat. No. 0 168 753 a truck which carries loads on its rear part may be connected in front of a safety car provided with a sensor for detecting obstacles. Upon contact with an obstacle, a pull-back device is operated to pull back the safety car against the front of the truck and to initiate the braking operation. Such an arrangement is expensive. Whereas it can be used to advantage for traveling around sharp bends, it does not permit a load-handling implement to be provided adjacent to such safety device.
In accordance with the publication mentioned last and, e.g., in accordance with Published German application No. 35 11 139, fork blades or loads are caused to trail in that they are provided at that end of the truck which is remote from the safety device. From that aspect the published German application which has been stated discloses an embodiment in which ultrasonic pulses are used.
In connection with the above-mentioned embodiment including a yieldable confirming structure, reference is also made to the prior art apparent from Published German application No. 32 19 247. In that case the safety arrangement comprises a mechanical shock absorber provided on a driverless truck. In a special embodiment that shock absorber comprises a compliant member and a pressure-responsive switching member. That shock absorber is also provided at the front end of the truck, which at that end does not have a special load carrier which serves to handle loads.
A specially designed collision-preventing stopping device of mechanical type is apparent from German patent specification No. 32 13 957. That device comprises a U-shaped contacting member, which is divided into sections and surrounds only a cavity and is adapted to be depressed into said cavity and is not operated in any relationship to a load carrier or to a load which has been picked up. In a special embodiment of that device, a deformation will initiate a braking but a lateral stability or the like will not be ensured.
A collision preventer may also be provided which uses infrared light. Reference is made in that connection to Published German application No. 35 01 598. In that case it is necessary to provide persons working in a plant with a transmitter, e.g., for infrared radiation, which is carried in a hip belt or on a helmet, and to provide the trucks with a receiver, which is tuned to the transmitter. Such a system is also expensive and is not reliable because there is no assurance that the protective device will always be worn.
From the large number of known designs it is apparent that it is a problem to prevent collisions of fork trucks, particularly of fork trucks having a front fork. The fact that the problem has not been solved thus far is believed to be due to the fact that the load is carried by the load carrier in front of the truck proper so that known collision preventers or the like which are disposed in front of the truck cannot operate reliably. It is also emphasized that the known safety arrangements using ultrasonics or infrared light are expensive and are subject to limitations from structural and space aspects.
From the aspects outline above, all known safety arrangements fulfill the required function but they are not related to a load which has been picked up so that their operation is not dependent on such load.